The mail scanner is probably not scanning your email again when you open it, rather the standard shield, I did some tests with OE and using the preview pane viewed some emails, the avast icon spins, but the total scanned in the Internet Mail provider doesn't increase, but it does in the Standard Shield scanned total. This however, souldn't cause that much of a hit unless your system is low on resources, cpu, ram, etc. that you didn't mention.
When something suddenly happens like this there has to be something that has changed otherwise the speed would be the same. Have you installed anything else recently?
What have you tried - to stop the scanning, e.g. the settings in